All plant roots require supplies of water, oxygen, and nutrients in order to grow and to provide needed water and nutrients so the plant tops can grow also. Plant growth media generally hold considerable amounts of water strongly enough to prevent its being lost by normal drainage, but loosely enough for the plants to take it from the medium. Satisfactory media generally are not saturated with water. Either they do not absorb enough water to become saturated, or they promptly allow enough water to drain from them to permit air to enter the system and fill the larger pore spaces. The air provides the needed supply of oxygen and also provides an exit for the carbon dioxide produced as the plant roots respire. When too little air is present, or when the air has too little communication with the atmosphere outside the growth medium, the oxygen supply becomes depleted to the point of deficiency and/or the carbon dioxide level builds up to the point of toxicity. Plant nutrient supply can be had by the nutrients being dissolved in water. The accepted principle for fertilizing plants is the same as watering and that is in batches. The water/aerating cycles in batches lead to the enormous waste of water and inefficiency of growth but can be learned with experience.
The batch approach on fertilizing crops is quite different, more complex and difficult to master. The first problem is that each plant has a different fertilizer need as to the total amount of fertilizer and the percentage and amount of each of the 14 elements (nitrogen, phophorus, potassium, magnesium, sufate, sodium, chlorides, iron, zinc, copper, boran, maganese, calcium and molybdenum). In all conventional methods of growing, the fertilizer is either stored in the soil and diluted and made available when water is added; or it is in the water. It is extremely important in this method of fertilizing that the correct total amount of fertilizer and the correct percentage and amount of each of the 14 elements be provided that particular plant as to its specific needs at that moment.
Hundreds of different blends of fertilizer in different percentage amounts of the elements have been developed to meet the needs for each commercial plant with further modifications for their different stages of growth. The roots have been subjected to and are forced to take the complete mix and cannot select whatever amount of each element the plant needs in batch feeding.
To complicate things even further, the pH of the soil and/or water effects the ability of the roots to absorb the fertilizer in general and/or specific elements.
The challenge of every farmer and hydroponic grower then is to apply to the soil or medium the correct proportions and amounts of each of the 14 elements of fertilizer, in the correct total volume, with the pH corrected as to meet each crop's requirements. What they apply is literally force-fed to the roots.
The usual plant growth medium is soil. Plants grow well in most natural soils because natural forces usually provide a satisfactory balance of water and air through the growing season. A variety of forces are at work including physical, chemical, and biological agents. Some important physical actions such as shifting caused by wetting and drying and freezing and thawing help form soils into granular porous media. The chemical and biological agents help to bind the granules together and release the needed plant nutrients.
Water and air share the pore space in granular porous media such as soil. A condition called "field capacity" is reached when applied water percolates downward into dry soil and stops with the lower part of the soil or its underlying material still dry. Depending on pore size, the pore space of a soil at field capacity commonly contains about 1/3 air and 2/3 water by volume. The air is in the larger pores. The smaller pores hold water so tightly that it will not move downward under the combined pull of gravity and of the dry soil below. Plant roots are able to withdraw approximately half of the water held in soil at field capacity. This "available water" serves the plant needs until the next rain or irrigation replenishes the soil water.
Some soils contain or are underlain by a layer with low permeability. A water table is produced in such soils when water percolates through the soil too rapidly to escape. The soil is saturated a little above as well as below the water table. Roots, other than those of a few unique plants such as rice, normally will not penetrate the saturated zone.
The saturated zone above a water table is called the "capillary fringe." Its thickness can be calculated if the pore sizes are known. The relationship is: thickness.times.pore diameter=0.3 cm.sup.2. Pores 1 mm (0.1 cm) in diameter hold water to a height 3 cm above the water table; those 0.5 mm in diameter to 6 cm above the water table, etc. This phenomenon is called capillary rise and results from the natural attraction between soil particles and water.
Researchers have found that soil needs to have enough air space to replenish the oxygen supply by diffusion at minimum rates between 5 and 25.times.10.sup.-8 g of O.sub.2 /cm.sup.2 per minute. Soil must usually contain at least 10% air space to achieve the minimum oxygen diffusion rate. Consequently, plants may be "drowned out" even if the water table is several inches below the soil surface. Water-loving vegetation may grow with a water table at a depth of one foot, but most plants will not. Farmers go to great expense installing ditches and tile lines to lower water tables to acceptable levels.
When soil is placed in a pot and the pot is placed in standing water, the condition is basically the same as that in a field soil with a water table at a depth equal to the height of the pot soil surface above its standing water level. Consequently, pots with plants growing in them normally cannot be allowed to stand in water. In fact, soil in pots will usually absorb and hold too much water if it is placed in water for a few minutes and then removed. Too much water applied to the soil surface has the same effect. Water will drain from the bottom of the pot intervals that the user deems appropriate. Often the user feels the soil in the pot to determine when more water is needed. The patents of Crowther, Iken, Klemm, Ludvig, Mieritz, Puccio, Reynolds, and Sigg* all use this method. Their patents tend to concentrate on appearance, tidiness, and convenience, or on cheapness and ease of transplanting. Several attractive and clever pot designs are represented in this group but none of them provide automatic control of the water supply. Some of them provide air passages around and through the walls of an inner container, but none of them discuss the principles of capillary rise, pore size, and aeration pore space. None of them specify any minimum amount or size of pores in the growing medium. Most of them mention soil as a growing medium and the rest don't mention what the growing medium would be. Ordinary soil lacks the properties on which the present system is based.
The wick method is used by Brankovic, Claveau, Millet, and O'Brien*. The wick method permits the use of a water reservoir that holds water at all times--a feature shared by the present system. Water is supplied to the growing medium through the wicks at rates controlled largely by the number, size, and porosity of wicks used. The control is based on a limited rate of transfer and cannot be allowed to reach equilibrium because that would result in saturation of the soil or other common medium (none of these patents specify a medium that would avoid this problem). The wick system works automatically when it is properly adjusted to the water demand. The rate of water transfer has only a small degree of self adjustment. Larger changes in plant size or even marked changes in atmospheric temperature and humidity require changes in the wick system. Too little wick capacity would cause the plants to wilt and too much would cause inadequate aeration for the plant roots. FNT *infra.
The vapor transport system is used only in Crater's patent. It is doubted that it would have enough capacity to prevent plants from wilting. It shares some principles with the wick method in that both are based on a rate of transfer method of control rate than on an equilibrium state.
The present system differs from all of the prior art patents in being an automated system with a control method based on a near equilibrium in the air-water relationships. It allows for a constant water supply to be maintained instead of the intermittent watering based on user judgment employed in conventional flower pots. It works automatically for all seeds and plants and eliminates the need for adjustments in the size, number, and type of wicks required to adapt the wick system to varying conditions. It has a large water transport capacity that is more than adequate for any needs because water is permitted to enter freely into the base of the pot. The capillary capacity of the entire cross section of the growth medium is available to transport water upward from the resulting water table. Excess watering causing inadequate aeration cannot occur even with zero water use because an equilibrium condition is reached in which the amount of aeration pore space is still adequate for plant growth needs. It is this equilibrium condition resulting from a growth medium containing enough large pores to remain aerated at equilibrium that makes the present system work. None of the previous patents uses or mentions this basic feature, nor would any combination of the previous patents produce this characteristic.